Method for improving the magnetic properties of silicon steel



United States Patent 3,142,591 1 METHOD FOR IMPROVING THE MAGNETIC PROPERTIES OF SILICON STEEL Jean Auguste Demeaux, Saint-Chely dApcher, Lozere, France, assignor to Compagnie des Ateliers & Forges de la Loire (Saint-Chamond, Firminy, Saint-Etienne, Jacob Holtzer), Paris, France No Drawing. Filed June 11, 1962, Ser. No. 201,253

Claims priority, application France June 16, 1961 Claims. (Cl. 148-3) My invention has for its object a method for the heat treatment of silicon steel ingots intended for magnetic purposes. My invention has for its object to improve the magnetic grade of the steel thus treated, which improvement is defined in particular by the measurement of the magnetic permeability and of the loss of electric power through hysteresis or possibly of properties associated with said loss of electric energy such as the residual induction or the coercive field.

Such so-called electric steels have silicon contents ranging between 0.5 and 5% and sometimes aluminium contents ranging between 0.05 and 0.5%, the other elements present in the steel being reduced to percentages as low as possible, for instance C=0.030%, Mn=0.050%, S=0.0l0%, P=0.015%, Cr=0.050%, Ni=0.050%, Cu=0.l00%, Sn=0.010%.

The present invention is not limited to particular means for producing the steel such as an electric furnace, an open hearth furnace, a converter or a combination of such means. It is applicable to silicon steel, whatever may be the manner of its obtention, after casting of the ingot.

A well-known fact for any one skilled in the art consists in that the magnetic grade of a silicon steel cannot be defined merely through a chemical analysis of the steel. The improvements brought by the present invention can be estimated only through comparison between the magnetic properties of a same steel, forming ingots obtained, on the one hand, according to the conventional procedures and, according to the invention, on the other hand, the methods of treatment following the casting of the ingot such as rolling, thermal treatment and the like being obviously the same for each type of steel ingot.

The method according to the invention is applicable to all electric silicon steels and more particularly to those in which the silicon contents range between 2 and 4.5%.

When the steel is prepared according to any metallurgical method, conventional practice, consists in casting it while still in a liquid condition into cast iron ingot molds inside which it is cooled and is speedily solidified. It is then subjected, according to its desired use, to different rolling and the like thermal operations which lead to the production of simple magnetic laminations, to simply directed laminae or markedly directed laminae.

The method according to the invention consists, immediately after the casting of the ingot, in cooling it uniformly throughout down to a temperature of 1,200 C. during at least two hours, counted from the moment of the solidification of the metal.

This improved method may be executed in different manners. In particular, the ingot may be cast into an ingot mold which is thermally insulated, or again, the ingot may be transferred after its casting inside a chamber subjected to a certain amount of heating so as to partly compensate for the losses of heat. The ingot may also be subjected after its casting to a heating through induction which provides the heat required for delaying the cooling. Said methods or a number of them may be associated.

The following comparative tests have been executed and are given by way of examples and by no means in a limiting sense.

43 samples of a same silicon steel, of which the silicon contents may however vary according to the case between Patented July 28, 1964 3.4 and 4% have been cast into ingots weighing each 560 kg, some being cast in conventional cast iron ingot molds and the others in sand molds of a thickness of 150 millimeters. The different ingots have been rolled and then reduced under similar conditions through hot and/ or cold rolling operations followed by annealing operations until a thickness of 35/ of a millimeter has been reached. The average losses of watts through hysteresis have been as follows:

For a magnetic induction of 10,000 gauss:

Watts Ingots cast in cast iron 0.88 Ingots cast in sand 0.82

For a magnetic induction of 15,000 gauss:

Watts Ingots cast in cast iron 2.39 Ingots cast in sand 2.24'

to Wit: the use of a sand mold rather than a cast iron mold results in a gain of 0.06 watt or 6.8% for 10,000 gauss and of 0.15 watt or 6% for 15,000 gauss.

Further tests have been executed with ingots weighing also 560 kg. cast inside an ingot mold of refractory concrete to millimeters thick and into the same ingot mold when heat insulated with a layer of vermicullite of a thickness of 80 to 120 millimeters, said ingot mold being heated to l,l00 C. previous to the casting of the ingots. For two steel castings having the following compositions: Si=3.60%, Al=0.30% and Si=3.70%, Al=0.40%, measurements concerning the hysteresis executed on toreshape samples cut out of the rough cast ingots have provided the following results:

This shows a substantial gain as concerns the loss of watts through hysteresis.

Similarly, the permeability measured on the same toreshaped samples shows, within the limits of the magnetizing fields ranging between 0.02 and 10 oersteds, an advantage for ingots cast in thermally insulated and/ or preheated ingot molds as compared with ingots cast in conventional cast iron molds. For instance, for a magnetizing field measuring 0.7 oersted, the permeabilities were as follows:

Conventional ingot molds 3,600 Ingot molds of refractory cement 4,750 Sand ingot molds 5,000

All these results show, on the one hand, the influence of the delay brought to the cooling of the ingots immediately after casting on the magnetic properties of the steel, the best insulated ingot providing the best results, while, on the other hand, they show that the improvement is substantial both for the steel of the ingots and for the laminae produced starting from said ingots, whatever may be the final product obtained under otherwise equal conditions.

The scientific explanation of these results cannot be reliably ascertained, but the various experiments made show that the improvement of the magnetic properties is reached whenever the ingot, starting from its temperature of solidification, which is equal to about l,450 C., is

J allowed at least two hours for cooling down to a temperature of 1,200 C., the curve of temperatures as a function of time decreasing uniformly and approximating the shape of a straight line.

When comparing with ingots of 560 kg. cast in cast iron ingot molds, it will be remarked that in this latter case, the outer wall is solidified in a few seconds, whereas solidification of inside of the ingot lasts about 12 minutes. The cooling down to 1,200 C. is performed in the outer section of the ingot in less than two minutes and in the inner section in about 15 minutes.

In contradistinction, in the case of an ingot which is thermally insulated by the sand forming the ingot mold, the temperatures remain practically uniform throughout the mass of the ingot of which all the sections are subjected consequently to the slow cooling referred to and are improved under the same conditions as concerns their magnetic properties.

Said uniformity of the temperatures of the ingot is an important further feature of the cooling method according to the invention, since it prevents any heterogeneity in the magnetic properties of the ingot and provides consequently better average properties for the products when completely finished, that is for the magnetic laminae in the more usual cases.

Said results may be obtained through other means than thermally insulated ingot molds. In particular, the ingot may be positioned with or without its ingot mold inside a thermally insulated chamber or a chamber the temperature conditions of which are controlled. The ingot mold may be subjected to the application of a predetermined amount of heat before the casting and during the cooling. The ingot may also receive directly external heat which makes up partly for the losses provided by the cooling; for instance, this may be obtained by an induction heating, while any other heating means may be resorted to. The different means referred to hereinabove may, of course, be associated in any desired manner.

What I claim is:

1. In the production of magnetic silicon steel with a silicon content ranging between 0.5 and 5% by casting the steel in an ingot mold, the improvement comprising slowly cooling the ingot through the temperature range of 14501200 C. at a cooling rate not exceeding about C. per hour.

2. A method as claimed in claim 1, in which the ingot mold is heat insulated.

3. A method as claimed in claim 1, in which the ingot mold is disposed in a heat insulated chamber.

4. A method as claimed in claim 1, in which the cooling rate is retarded by application of heat to the ingot.

5. A method as claimed in claim 4, in which heat is applied to the ingot by induction heating.

Corson Apr. 5, 1932 Goodsell Nov. 25, 1952 

1. IN THE PRODUCTION OF MAGNETIC SILICON STEEL WITH A SILICON CONTENT RANGING BETWEEN 0.5 AND 5% BY CASTING THE STEEL IN AN INGOT MOLD, THE IMPROVEMENT COMPRISING SLOWLY COOLING THE INGOT THROUGH THE TEMPERATURE RANGE OF 1450-1200*C. AT A COOLING RATE NOT EXCEEDING ABOUT 125*C. PER HOUR. 